Method of processing a digitized workpiece, particularly a three-dimensional model of a dental prosthetic item to be produced therefrom, and apparatus therefor

ABSTRACT

In a method of processing a digitized workpiece ( 1 ), especially a three-dimensional model of a dental prosthetic item to be produced therefrom, by altering the surface data of the workpiece by means of a tool which acts on a local zone of action ( 3 ) on the surface ( 2 ), a change in height of said surface ( 2 ) is effected in said zone of action ( 3 ) of said tool when the tool is applied to said surface, the said zone of action of said tool covering a modified region ( 5 ) showing a substantially constant change in height (t) over the zone of action ( 3 ) and also a transition region ( 4 ), in which the surface of the modified region ( 5 ) merges into the surface surrounding said zone of action ( 3 ).

The invention relates to a method of processing a digitized workpiece,particularly a three-dimensional model of a dental prosthetic item to beproduced therefrom, by altering the surface data of the workpiece bymeans of a tool which acts on a local zone of action on the surface.

A design being processed by CAD is not constrained by the limitations ofthe physical characteristics of materials and tools, indeed, theoperation of the tool can completely free itself from thesecharacteristics and overcome their disadvantages. It is desired, inparticular, to improve the operability thereof.

Existing dental CAD/CAM systems offer no free-form tools that go beyondthe techniques used in conventional dental technology. Thus thesefree-form tools, like those used in manual processing of physicalmodels, have the disadvantage that the fine structures of the surfaceare obliterated due to the application of wax.

DESCRIPTION OF THE RELATED ART

In dental CAD/CAM systems, various tools are used for designing modelsfor the fabrication of dental prosthetic items. Using these tools, thedigital model of the dental prosthetic item can typically be modified ona line or point basis. For example, the equatorial line can be changed,the whole model can be rescaled or displaced, individual cusps can bemade higher or lower, or the site of a cusp point can be shifted.

Furthermore, tools are used with which one can freely change the shapeof the surface and which are derived from techniques of conventionaldental technology, for example, the application of drops of wax or theremoval of material with a knife, both realized of course in a digitalform.

The line-based or point-based tools are usually easy to use, but theyhave the disadvantage that the user can only insufficiently andindirectly influence the surface between the lines or points.

The tools derived from conventional dental technology are typicallyoriented toward the characteristics of mechanical tools like, forexample, a knife, or toward the characteristics of wax, for example itsdrop size or its temperature.

It is an object of the invention to provide the user of CAD/CAM systemswith a tool that gives him additional possibilities in designing theshape of models.

SUMMARY AND OBJECTS OF THE INVENTION

The method proposed herein for processing a digitized workpiece,especially a three-dimensional model of a dental prosthetic item to beproduced therefrom, by altering the surface data of the workpiece bymeans of a tool operating on a local zone of action on the surface,consists in applying the tool to the surface to produce a change inheight in the zone of action of the tool, the said zone of action of thetool covering a modified region showing a substantially constant changein height over the said zone of action, as well as a transition regionin which the surface of the modified region merges into the surfacesurrounding the zone of action.

The tool proposed herein enables the user to apply or remove a thinlayer of material of substantially constant layer thickness (hereafterreferred to as a “change in height”) at an arbitrary point on thesurface. The size of the region is adjustable. By using a repeatfunction when moving the tool over the same place on the surface, theuser can build up and/or clear away tooth structures in a simple way,without obliterating the fine structural features.

By decoupling from the paradigm of conventional dental techniques, newpossibilities for the design of dental prosthetic items arise. The usercan effect better control of the change in shape than with currentlyavailable tools whilst retaining existing fine structural features.

The ratio of the change in height in the zone of action to the extent ofthe zone of action is at most 1:50, and preferably between 1:100 and1:200. It is advantageous for the ratio of the thickness of the changein height to the extent of the zone of action to be 1:140. This allowsfor sufficiently fine processing without destroying fine structuralfeatures.

It is advantageous for the ratio of the width of the transition regionof the zone of action to the change in height to be between 1:1 and20:1. This allows for a sufficiently smooth transition.

The position of the tool can be advantageously represented by a cursorthat is movable over the image of the surface of the workpiece, andwhich corresponds to a selected region of the surface. The selectedregion can be independent of the zone of action of the tool.

The surface covered by the zone of action will be changed by means ofthe tool in the direction of a vector normal to the surface.

The direction of the change of the surface in the zone of action can bederived, in particular, from a normal vector averaged over an evaluationzone of the tool. The evaluation zone can span all or a part of the zoneof action, the modified region, or the selected region.

It is preferable that the normal vector be calculated from a selectedregion in which the ratio of the selected region to the entire region isbetween 1:5 and 1:200.

The change in height can consist of either an augmentation or areduction of material, and a means for choosing the direction of thechange in height is provided.

The choice of the direction of the change in height can be indicated byvarious representations of the tool. The currently activated mode of thetool is then immediately recognizable to the user from the monitorscreen.

It may be advantageous to visibly distinguish the region alreadyprocessed by the tool from the representation of the surface of themodel. The user will thus know which region of the surface has alreadybeen processed. Furthermore, frequently processed surfaces can be showndifferently from once processed surfaces, for example by coloredhatching.

It is advantageous for the size of the zone of action to be variable.For that reason, means for displaying the size of the zone of action areprovided. In particular, the size of the zone of action can be displayedby colored marking on the surface of the model. The user can enlarge ordiminish the zone of action.

It is preferred that the degree of the change in height will depend onthe extent of the zone of action. In particular, the change in heightcan decline with increasing size of the zone of action.

Another object of the invention is a device having means for carryingout the method.

BRIEF DESCRIPTION OF THE DRAWINGS

The method of the invention will be explained below with reference tothe following diagrams, in which

FIG. 1 shows a digitized workpiece in the form of a three-dimensionalmodel of a dental prosthetic item, indicating the region beingprocessed;

FIG. 2 shows a section of the workpiece of FIG. 1 in the zone of actionof the tool;

FIG. 3A shows the digital model of the dental prosthetic item of FIG. 1with the zone of action 3 of the tool on the surface 2;

FIG. 3B is an illustrative sketch of the zone of action of the tool aswell as other relevant regions;

FIG. 4A shows a segment of the workpiece surface and the zone of actionof the tool;

FIG. 4B shows a segment of FIG. 4A;

FIG. 5 shows an enlargement of the zone of action of the tool of FIG.4A, for determining the direction of the change;

FIG. 6A shows in detail a modification of a surface made with the tool;

FIG. 6B shows in detail a modification of a second surface made with thetool;

FIG. 7 shows in detail the profile of the change in a transition region;

FIG. 8A shows a first profile of a change in a selected region with avarying normal vector;

FIG. 8B shows a second profile of a change in a selected region with asharply varying normal vector; and

FIG. 9 shows a diagrammatic relationship between the area of the zone ofaction of the tool and the change in height.

DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

FIG. 1 shows a digitized workpiece in the form of a three-dimensionalmodel of a dental prosthetic item for production therefrom, which is tobe altered by the user. The model is shown on the monitor of a computer.

During processing, the surface 2 of the model of the dental prostheticitem, present in the form of digital data, will be processed by a toolwhich influences the design, ie, influences these data. The tool acts ona local zone of action 3 on the surface 2. The zone of action 3 is shownas a circle, but can have other suitable shapes.

Input means, not shown in the figure, are provided for moving the zoneof action 3 of the tool over surface 2. Examples thereof are a computermouse or a digital pen plotter which controls a cursor 11 or 12. Thetool is initially free to move over the surface of the representationand can be activated as needed. This happens for example by amouse-click at a particular place or by clicking and holding a mousebutton while at the same time dragging it from a starting point.

The choice of the direction of the change in height is indicated bydifferent depictions of cursor 11 or 12 of the tool; cursor 11, shown asa hand with a plus sign, indicates material deposition, and cursor 12,as a hand with a minus sign, indicates material removal. The currentlyactivated mode of the tool is then immediately recognizable to the userfrom which of the cursors 11 and 12 is displayed. Contrary to thefigure, only one of the two cursors 11 or 12 will be displayed to theuser at any one time.

The direction of the change in height may be chosen by a selectiondevice. Switching between deposition and removal of material to producea change in height of the surface can, for example, be done by pressingthe space bar on a computer keyboard.

FIG. 2 shows the dental prosthetic item in transverse cross-section,taken through the zone of action 3 of FIG. 1. Note the shape of surface2, which is calculated from the surface data and displayed, and notealso the change carried out on the surface within the zone of action 3by means of the tool in deposition mode, as indicated by the cursor 11.

In FIG. 3A the digital model of the dental prosthetic item of FIG. 1 isshown with the zone of action 3 of the tool on the surface 2 around thecursor 11, the extent of the zone of action 3 being indicated by markingthe surface 2, here by using a different color. Representation using apatterned texture, like hatching, is also possible. The user can enlargeor reduce the zone of action 3. For this purpose, means for changing thesize of the zone of action are provided.

In FIG. 3B, an illustrative sketch, shows the zone of action 3, as wellas other relevant regions, on a subregion of surface 2. The zone ofaction is shown as a circle, but can also take on other appropriateshapes. In principle, all smoothly-bordered surfaces are possible, withelliptical shapes being preferred, as they present no corners.

The zone of action 3 has an extent D, and surrounding the zone of action3, no change to the surface 2 takes place.

A transition region 4 is located within the zone of action 3 and has awidth b; in the present example it is concentric with the zone of action3 and centered about a midpoint M.

The transition region 4 merges into a modified region 5 which shows asubstantially constant change in height.

The position of the tool is represented by a cursor, as in FIG. 3A, towhich a selected region 6 is assigned. This selected region, of diametera, is independent of the zone of action of the tool; however, in theexemplary embodiment, it is concentric with the zone of action 3.

When using line widths, brushes, or sprays as image-processing tools, itis known that the diameter of the selected region fits the diameter ofthe zone of action, so that, for example, the line width, the brushwidth, or the width of the spray stream entails a change in the size ofthe selected region of the cursor. Similar presetting can be made by theuser, with the possibility of changing it.

In order to make the surface area of the regions that have already beenprocessed clearly visible, the zone of action 3 with its extent D can behighlighted with color against surface 2, without immediately displayingthe change in height.

Alternatively, it is possible to show the effect of the tool byrecomputing the processed regions and displaying the new, changed model,since the display of the model of the dental prosthetic item in FIG. 1already has reflections and shadings, and changes may be determinedsimply by altering the reflections and/or shadings as a consequence ofthe application of the tool. The intermediate step involving aninitially two-dimensional representation is then superfluous.

Finally, an evaluation region 7 is provided, which extends over adistance n, and within which a normal vector n₀, determining thedirection of the change in height, will be constructed from the vectorsnormal to the surface 2 within the evaluation region, for example byaveraging over the evaluation region. In the exemplary embodiment, theevaluation region 7 is also concentric with the zone of action 3, butits extent n is independent of the zone of action 3 and the selectedregion 6, so that even using a cursor with a small evaluation region 6and a setting with a large zone of action 3, the evaluation region 7 isunchanged. Obviously the evaluation region can also be adjusted to fitthe zone of action. Experience has shown it to be practical to make theevaluation region at least as large as the selected region but notlarger than the zone of action.

In FIG. 4A the division of the surface 2 into different regions, asdefined in FIG. 3, is shown diagrammatically in a sectional drawingfollowing FIG. 2.

Here the dental prosthetic item 1 is shown with its surface 2, on whichthe zone of action 3 is indicated by virtual boundary lines separatedfrom each other by a distance D. The boundaries of the zone 3 are fixedby a distance from a midpoint M, which is part of the surface 2. Thenormal vector nM will be determined at the midpoint M, for example bydetermining the tangent t_(M) to the surface 2 at the point M.

The value of the normal vector is of secondary importance in the presentmethod, as the change in height is definable as a pre-settingirrespective of the curvature of the surface, and it remains the sameduring processing of different locations on the surface 2. In this waythe user will achieve an estimable change in height by using the tool.

Also shown are the tangents t₁, t₂ as well as their normal vectors n₁,n₂ at the edge of the zone of action 3 of the tool. From these it isclear that the normal vector, in the case of a curved surface 2, takeson different directions over the extent D of the zone of action.However, for the determination of the direction of the change in heightin the zone of action 3, and thus of the normal vector n₀, only theevaluation region, with an extent of n, will be used; this is shown indetail in FIG. 4B, with the tangents t₃, t₄, as well as their normalvectors n₃, n₄, on the border of the evaluation region having an extentof n.

This is further clarified in FIG. 5. Here again, on the surface 2 of thedental prosthetic item 1, there is shown the zone of action 3 with itsextent D about the midpoint M.

Over the evaluation region, an average is made of the direction of thenormal vectors nm, n₃, n₄, which are derived from the tangents t_(M),t₃, t₄ to the surface 2. The normal vector n₀ resulting from thisaveraging coincides in the exemplary embodiment with n_(M). Thedetermination of a normal vector from a selected region with an extentsmaller than the extent of the zone of action yields obvious advantagesas regards computation effort. In the event that sufficient computingpower is available, the normal vector can be evaluated over a largerregion, if desired.

On surfaces showing marked local curvature, a larger region leads togreater smoothing of the normal vector and, on the whole, to a morestable procedure. On slightly curved surfaces, the smoothing leads tosimilar directions of the normal vector, in spite of different regionsof the surface having different normal vector directions being selected,so that the direction of the change is not as controllable as when asmaller evaluation region is used.

In FIG. 6A the change of the surface 2 through the use of the tool isshown in detail. Within the zone of action 3, excluding the transitionregion 4 of extent b, a constant change in height t of the surface 2 ofthe dental prosthetic item 1 is carried out over the entire modifiedregion 5 in the direction of the normal vector n₀. This means that alayer having a thickness t is added over the entire modified region 5 ofsurface 2, at every point thereof, in the direction of the normal vectorn₀.

FIG. 6B shows the change in the surface 2 in the region of a fissurethat lies surrounding the zone of action 3. A constant change in heightt of the surface 2 is carried out over the entire modified region 5 inthe direction of the normal vector n₀ except in the transition region 4of extent b. This means that a layer having a thickness t is added overthe entire modified region 5 of surface 2, at every point thereof, inthe direction of the normal vector n₀. The normal vector is calculatedfrom the evaluation region 7, which has an extent n.

In FIG. 7, the transition region 4 between the modified region 5 and theunchanged surface 2 is shown. Within the transition region 4 of extentb, the surface 2 is changed such that both boundaries of the transitionregion 4 merge smoothly into the respective adjoining regions. For thispurpose, the tangent t₂ to the surface 2 at the boundary of the zone ofaction 3, and the tangent t₅ to the surface 2 at the inner boundary ofthe transition region 4 are found and the profile of the surface 2 isdefined. Due to the constraints imposed by smooth tangential merging,the surface 2, shown in section, has an inflection point. The transitionregion can, for example, take the shape of a Gaussian curve.

For the sake of completeness it is pointed out that the direction oftangent t₅ is the same as that of tangent t₆, which, at the innerboundary of the transition region 4, is displaced by a layer thickness tdue to the change in height in the direction of the normal vector n₀.

FIG. 8A shows the results of the tool in a region with a sharplychanging normal vector. Such changes occur, for example, near a fissure2 in a dental prosthetic item. Here again, proceeding from a midpoint Mover the zone of action 3, with the exception of a transition region 5,a change of the surface 2 is brought about in which a change in heightof constant thickness t is effected in the direction of a normal vectorn₀. The normal vector n₀ was determined over the evaluation region 7,with the fissure 2 lying within the evaluation region.

FIG. 8B shows the results of the tool in a zone of action having asharply changing normal vector, with the evaluation region lying outsidethe fissure 2. Here again, proceeding from the midpoint M over the zoneof action 3, with the exception of a transition region 5, a change ofthe surface 2 is also brought about, in which a change in height ofconstant thickness t is effected in the direction of a normal vector n₀.It appears as if the part on the right-hand side has duplicated itself,but what has actually occurred here is again a shift by approximatelythe thickness t toward the transition region. There the modified region5 merges into the transition region 4 and into the surface 2 surroundingthe zone of action 3.

One can see from FIGS. 8A and 8B, that the fissure 2 substantiallyremains. This is a decisive advantage of this tool over known free-formtools.

In FIG. 9, a possible relation between the extent D of the zone ofaction of the tool and the change in height t is shown diagrammatically,for which it always holds that the extent D_(a) of the selected zone ofaction is larger than the selected thickness ta. In practice, it hasbeen shown that it is preferable for the extent D of the zone of actionto be much larger than the thickness t. One may furthermore note fromFIG. 9, that with increasing values of the extent D of the zone ofaction, the thickness t of the change in height is reduced, so that ifthe user chooses a large zone of action, the change in height t will besmaller than when the user chooses a smaller zone of action. In practicehowever, it has been found that the extent D of the zone of action neednot have any influence on the thickness t, and that therefore thethickness t can be independent of the extent D.

The exemplary embodiment is clarified by the following numerical values.For a change in height by a layer thickness t of 10×10⁻⁶ m, one obtains,at a ratio of 1:140, a width of 1400×10⁻⁶ m for the zone of action. Inthis case, the transition region, at a ratio of 10:1 to the layerthickness, will have a thickness of 100×10⁻⁶ m, whereby a sufficientlyfine transition is assured.

In an apparatus for processing a digitized workpiece, especially athree-dimensional model of a dental prosthetic item to be producedtherefrom by altering the surface data of the workpiece by means of atool which acts on a local zone of action (3) on the surface (2), meansare provided which, when the tool is applied to the surface in a zone ofaction (3) of the tool, bring about a change in height of the surface(2), which zone of action (3) includes a modified region (5) showing achange in height that is substantially constant over the zone of action(3) and a transition region (4) in which the surface of the modifiedregion (5) merges into the surface surrounding the zone of action (3).

These means include algorithms for changing the surface data and meansfor selecting that region of the surface which is to be processed.

This device can further include the features which are necessary forcarrying out the method of the invention, as described above.

1. A method of processing a digitized workpiece, particularly athree-dimensional model of a dental prosthetic item to be producedtherefrom, by altering surface data of the workpiece by means of a toolwhich acts on a local zone of action on the surface, wherein when thetool is applied to said surface a change in height of said surface iseffected in said zone of action of said tool, said zone of action ofsaid tool covering a modified region showing a substantially constantchange in height over the zone of action and also a transition region,in which the surface of the modified region merges into the surfacesurrounding said zone of action.
 2. A method as defined in claim 1,wherein said change in height of said zone of action bears a ratio tothe extent of said zone of action of most 1:50.
 3. A method as definedin claim 2, wherein said transition region of the zone of action has awidth which bears ratio to the change in height of from 1:1 to 20:1. 4.A method as defined in claim 1, wherein a position of the tool isindicated by a cursor which can be moved by control means over thedisplay of the surface of the workpiece and corresponds to a selectedarea on said surface.
 5. A method as defined in claim 1, wherein use ofsaid tool changes the surface of said zone of action in the direction ofthe vector normal to the surface.
 6. A method as defined in claim 5,wherein the direction of the change of said surface in said zone ofaction is determined by a normal vector averaged over the evaluationrange or over the modified region of the processing tool.
 7. A method asdefined in claim 4, wherein said normal vector of said selected area iscomputed, the selected area bearing a ratio to the entire region of from1:5 to 1:200.
 8. A method as defined in claim 1, wherein said change inheight can entail addition or removal of material and choosing thedirection of said change in height is made possible by selection means.9. A method as defined in claim 8, wherein the selection of thedirection of said change in height is indicated by differentrepresentations of the free-form tool.
 10. A method as defined in claim1, wherein said zone of action of the free-form tool is visiblydistinguishable from the representation of the surface of the workpiece.11. A method as defined in claim 1, wherein the size of said zone ofaction is modifiable, and including means for indicating the size ofsaid zone of action.
 12. A method as defined in claim 1, wherein theamount of change in height is dependent on the extent of said zone ofaction.
 13. A method as defined in claim 11, wherein the change inheight diminishes with increasing extent of the zone of action.
 14. Anapparatus for processing a digitized workpiece, especially athree-dimensional model of dental prosthetic item to be producedtherefrom, by altering surface data of the workpiece by means of a toolwhich acts on a local zone of action on the surface, wherein means areprovided which, when the tool is applied to the surface in a zone ofaction of said tool, bring about a change in height of said surface,which zone of action includes a modified region showing a change inheight that is substantially constant over said zone of action and atransition region in which the surface of the modified region mergesinto the surface surrounding said zone of action.
 16. A method asdefined in claim 2, wherein said ratio is 1:100 to 1:200.